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May 26, 2017 - ABSTRACT: It has been challenging to separate oil from oil/water emulsions with droplet size less than 1 μm using conventional porous ...
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Separation of Oil-in-Water Emulsions Using Hydrophilic Electrospun Membranes with Anisotropic Pores Weerapha Panatdasirisuk,†,§ Zhiwei Liao,‡ Thammasit Vongsetskul,§ and Shu Yang*,†,‡ †

Department of Materials Science and Engineering and ‡Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States § Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Road, Ratchathewi, Bangkok 10400 Thailand S Supporting Information *

ABSTRACT: It has been challenging to separate oil from oil/water emulsions with droplet size less than 1 μm using conventional porous membranes. Membranes with small pores are preferred, but the trade-off is a dramatic reduction of volumetric flux. Here, we prepared membranes from electrospun polycaprolactone (PCL) fibers with high porosity (∼88%). When the membranes were stretched uniaxially at different strain levels, the pores became anisotropic with an aspect ratio (pore length/width) up to 5.3 ± 3.0. To improve their wettability, we added Tween 80, a hydrophilic surfactant, to PCL solutions for electrospinning. The modified PCL membranes showed excellent mechanical properties with a tensile strength at 6.59 ± 1.67 MPa and the elongation at break up to 130 ± 21%, warranting their use as free-standing separators. We narrowed the pore gap while maintaining the high porosity by stretching the membranes. Scanning electron microscopy (SEM) images of the stretched membranes show changes of pore geometry without altering the fiber size and fiber network integrity with strain up to 80%. The anisotropic membrane could exclude oil from oil-in-water emulsion droplets with a diameter as small as 18 nm without reduction of the volumetric flux in comparison with the nonstretched one.

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INTRODUCTION Oil spill, an important environmental threat, adversely affects photosynthesis and respiration of aquatic plants and animals because the thin layers of oils cover water surface and prevent sunlight and oxygen to pass through. Moreover, oil can penetrate into the plumage and bird fur, resulting in breaking down the thermal insulating capabilities of feather.1 Free oil can be removed by skimming, but this method is inefficient for emulsified oil, which is much more stable than simple oil and water mixtures, especially when droplet sizes become smaller. Oil absorbing materials, coagulants, or de-emulsifiers could be used to remove the emulsified oil, but they typically suffer from their low separation efficiency and will generate secondary pollution.2 In comparison, membranes potentially offer high separation efficiency, no generation of secondary pollutant, and low energy cost.1,3−6 In the past decade, various types of membranes have been prepared.7 For example, porous silica nanofibrous membranes with superhydrophilicity and underwater superoleophobicity showed their ability to effectively separate oil/water microemulsions with an average droplet size of 3.82 μm.8 Nevertheless, it remains challenging to separate emulsions with diameters less than 1 μm. In a typical membrane separation, selectivity is based on the principle of size exclusion. To separate emulsion droplets with diameter less than 1 μm, it requires membranes with pore size in nanoscale. Meanwhile, it has been suggested that the volumetric flux is proportional to the pore radius to the power of 3.9 Thus, the flux of the membrane will drop dramatically when decreasing the pore size. Methods such as applying © XXXX American Chemical Society

pressure and reducing the thickness of the membrane have been used to increase the flux. To do so, it demands membranes with high mechanical strength. Holdrich et al. show that membranes with slotted pores (pore size of 10−20 μm) had lower membrane resistance and higher volumetric flux compared to those with circular pores.10 Nevertheless, little is reported about the impact of pore geometry to filtration efficiency and volumetric flux as the pore size shrinks to nanoscale. For microfiltration and ultrafiltration, the common techniques to fabricate polymeric membranes are phase inversion, interfacial polymerization, track etching, and electrospinning.11 Among them, electrospinning provides nonwoven membranes with high porosity as well as a variety of pore morphology and chemistry.12 Besides pore morphology and pore density, surface wettability is another critical parameter. In order to reject oil, the membrane should be (super)hydrophilic and underwater (super)oleophobic. Oil phase can be repelled by a thin layer of water impregnated within the membrane; otherwise, oil fouling will lead to the reduction of flux and lifetime of filter membranes.13−16 The (super)hydrophilicity and underwater (super)oleophobicity can be achieved by a combination of surface chemistry and roughness.17 However, hydrophilic polymers often have poor mechanical properties under water as they will swell and disintegrate over Received: April 4, 2017 Revised: May 10, 2017

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DOI: 10.1021/acs.langmuir.7b01138 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a, b) SEM images of the PCL membranes without (a) and with (b) Tween 80. (c, d) Water wettability of the corresponding PCL membranes. (a, c) 13% w/v PCL without Tween 80. (b, d) 13% w/v PCL mixed with 2% v/v Tween 80. (e) Illustration of the physical grafting of Tween 80 in a PCL fiber via entrapment. The liquid flow rate was controlled at 1.5 mL/h using a home-built programmable syringe pump. A rotating drum (diameter of 10 cm, Tong Li Tech, China) was placed at a distance of 10 cm from the needle for collecting the fibers at a rotating speed of 50 rpm over 2 h. To prepare PCL membranes of elongated pores, the membrane was gripped between two acrylic bars on both sides, followed by mechanical stretching to different strains at room temperature as shown in Figure S2. Emulsion Preparation and Filtration. The mixture of 30% v/v Span 80 and 70% v/v Tween 80 was prepared as emulsifiers following the procedure reported in the literature.23,24 1% v/v hexadecane and 0.1% v/v emulsifier in DI water were mixed by vortexing for 1 min. Hexadecane was dyed by Sudan red for better visualization. The freestanding PCL membrane was firmly sandwiched between a pair of glass cylinders (diameter, 15 mm). For filtration, the feed emulsion was poured into the top cylinder and passed through the PCL membrane under gravity. The permeate was collected to evaluate filtration efficiency and particle size. The flux was calculated from the volume of permeate per time. Characterizations. The morphology of the PCL membranes was characterized by a high-resolution scanning electron microscope (HRSEM, JEOL 7500F, USA). To measure the average pore size before and after stretching, we used ImageJ 1.48v software to process the SEM images. The mechanical property was investigated by Instron (Model 5564). The specimens were cut into a rectangular shape (3 cm × 0.5 cm) and pulled at a speed of 10 cm/min. The measurement was averaged over five tests per sample. The contact angle was measured using an automated goniometer (Ramé-hart, USA) over three fresh spots per sample. The size of emulsion droplets before and after the filtration was characterized by dynamic light scattering (DLS) from a Zetasizer Nano S (Marvern Instruments, UK). To evaluate the oil content in feed and permeate, chemical oxygen demand (COD) analysis was carried out on by the reactor digestion method. The feed or permeate was diluted and added to the reaction vial containing a digestion solution of chromic acid, mercuric sulfate, silver sulfate, and sulfuric acid that can measure the oxygen consumed in the range of 3− 150 mg/L during the reaction. Before the mixture was placed into the spectrophotometer (DR/2000, HACH, USA) and set to program number 435 for COD measurement to obtain the oil concentration, the sample was heated to 120 °C for 2 h and cooled to room temperature.

time. Composite membranes from electrospun poly(vinyl alcohol) (PVA) and polyacrylonitrile (PAN) fibers have been assembled for wastewater treatment by taking advantage of the hydrophilicity of PVA.18 To enhance the strength of the hydrophilic fibers and prevent their dissolution in water, PVA is cross-linked by glutaraldehyde (GA, 50 wt % in water).19 Nevertheless, their long-term durability in water remains to be seen. Therefore, treating hydrophobic polymers to be hydrophilic may offer a better alternative to achieve underwater (super)oleophobicity while maintaining mechanical integrity. For example, hydrophobic polymer, poly(L-lactide), was blended with poly(3-hydroxybutyrate-co-4-hydroxybutyrate) to increase the membrane hydrophilicity for separation of simple oil/water mixtures.20 A large quantity of copolymer is needed to achieve hydrophilicity, which is not only expensive but also potentially alters the mechanical strength of the membrane. Mixing hydrophilic chemicals in polymer membranes is another option, but it often causes phase separation, which makes it difficult to electrospin.21 In this study, we prepared membranes from nonwoven electrospun fibers of polycaprolactone (PCL). PCL is relatively hydrophobic. To improve the wettability of the membrane surface, we introduced Tween 80, a nonionic surfactant derived from polyethoxylated sorbitan and oleic acid, in PCL solution before electrospinning. We then uniaxially stretched the PCL− Tween 80 membrane (with a fiber diameter of 259 ± 80 nm) up to 80% strain, changing the pore aspect ratio (length vs width) from 2.5 ± 1.9 to 5.3 ± 3.0. The obtained membrane was evaluated for filtration of stable oil-in-water (O/W) emulsions. We found that O/W emulsion droplets with a diameter as small as 18 nm could be separated from the stretched membrane while the volumetric flux was maintained.

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EXPERIMENTAL SECTION

Materials. PCL (average Mn = 80 000 g/mol), acetic acid (99.7%), Tween 80, Span 80, hexadecane (97.0%), and Sudan red were purchased from Sigma-Aldrich, USA. Formic acid (88%) was purchased from Fisher Scientific, USA. Chemicals were used without further purification. PCL Membrane Preparation. A solution of 13% w/v PCL and 2%v/v Tween 80 was prepared by dissolving PCL pellets and Tween 80 in a mixed solvent of acetic acid and formic acid (3:1 v/v). The mixture was stirred at room temperature for 3 h to obtain a clear solution. The electrospinning was carried out according to the literature.22 In brief, the solution was loaded into a 5 mL syringe (Fisher Scientific, USA) with an 18G blunt needle (CML Supply, USA). A positive voltage (16 kV) supply was connected to the needle.

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RESULTS AND DISCUSSION PCL is strong and stretchable. Because it is relatively hydrophobic, for “water-removing” type filteration,17 that is, separating oil from the oil/water mixtures or emulsions, chemical treatment or blending PCL with hydrophilic polymers has been applied to improve the hydrophilicity of electrospun PCL fibers.20,22 The latter is simpler and has a lower risk of B

DOI: 10.1021/acs.langmuir.7b01138 Langmuir XXXX, XXX, XXX−XXX

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Langmuir damaging fibers via chemical treatment. However, the blended components should be miscible with the PCL solution. Otherwise, phase separation occurs, and thus it will obstruct the electrospinning process. Moreover, without a cross-linking process, the hydrophilic components could be washed off by water. Another concern is that adding new materials for electrospinning could alter the morphology of the fibers, which in turn could have a negative effect on the mechanical properties. Here, we added a small quantity (2% v/v) of Tween 80, a nonionic surfactant into PCL in a mixed solvent of acetic/ formic acid (3:1 v/v). A clear solution was obtained. The electrospinning process was performed under an ambient condition, following the reported procedure22 without phase separation. The obtained membranes were white, uniform, and flexible. The size and morphologies of PCL fiber network with and without Tween 80 were nearly identical (Figure 1) with an average fiber diameter of 259 ± 80 and 258 ± 50 nm, respectively. However, addition of Tween 80 dramatically changed the wettability of PCL membrane; the one with Tween 80 could be wet completely by water (Figure 1c vs Figure 1d). Since the morphology of the fibers was similar, the change of wettability must be attributed to fiber surface modification. Figure 1e illustrates the proposed mechanism of physical grafting Tween 80 within the PCL fibers. Once the polymer jet is ejected from the tip of the needle, solvent begins to evaporate. The migration of solvent drags along the small molecules, Tween 80 toward the fiber surface. In this study, acetic acid and formic acid were used as solvents, which were much more hydrophilic than PCL chains. Therefore, their migration toward fiber surface would be the main driving force of the rearrangement of hydrophilic heads of Tween 80 toward the air/fiber interface, while moisture in the air would promote the rearrangement of Tween 80. In experiments, we did not see any difference in wettability on fibers obtained in different humidity, ranging from 40% to 60%. Tween 80 molecules are entangled within the fibers upon a complete evaporation of the solvent, rendering the PCL fiber surface hydrophilic. Previously, we have applied a similar approach to modify the surface of polymer pillars from relatively hydrophobic SU-8 with Pluronic, a triblock copolymer of ethylene oxide and propylene oxide.25 To test whether Tween 80 could stay on fibers and the fibers remain hydrophilic during the filtration, we washed PCL membranes with a large amount of water. Before washing, water spread on PCL membranes instantly. The wettability dropped after the first washing. It took ∼9−10 s for water to spread on the membrane completely. Afterward, the wettability remained nearly constant even after washing 10 times and immersed in water overnight (Figure S1), comparable with the wettability by blending PCL with 50% of block copolymers.20 Here, mixing a small quantity of Tween 80 with PCL for electrospinning offers a much simpler method to improve the hydrophilicity of the filtration membrane without altering the fiber morphology or the physical properties. From now on, the PCL membranes used in this study are PCL grafted with Tween 80. To separate oil from oil/water (O/W) emulsions on the basis of size exclusion, we should prepare the membrane with pore size smaller than the emulsion droplets. Typically, the pore diameter of electrospun membrane scales with the fiber diameter due to the packing constraints during the fiber deposition by conventional electrospinning.26 Reducing the viscosity of the electrospinning solution leads to fibers with a

smaller diameter.27 However, it also reduces the polymer chain entanglement. Therefore, it becomes more difficult to jet the polymer solution in the form of fibers during the electrospinning process. Meanwhile, as mentioned earlier, reducing fiber size and porosity will dramatically drop off the flux as the trade-off. It has been shown that membranes with micrometer-sized slotted pores have higher volumetric flux than those with circular pores.10 It is attributed to a lower membrane resistance per unit area R slot = circular pores, R circular

32 πd 2l

1 N

( ) for slotted pores vs that with = ( ) , where d is the slot width or 24 d3

1 N

pore diameter, N is the number of pores per unit area, and l is the depth of the pore. The volumetric flux Q = ΔP/(ηR), where ΔP is the pressure drop through the membrane and η is the dynamic viscosity of the liquid. Nevertheless, little is known about the impact of pore geometry on filtration efficiency and flux as the pore size reduces to nanoscale. In addition, the comparison of volumetric flux reported in the literature is made between well-defined slotted pores and the isolated, tracketched circular pores. There are a number of ways to align the fibers (or pores) in the membrane network such as the use of parallel plates, varying the spinning speed, and stretching the random mesh. Here, we mechanically stretched the PCL membranes at different strain levels to change the pore shape, which reduced the pore gap without altering the overall porosity. First, we investigated the mechanical properties of the obtained nonwoven PCL membrane, whether it was possible to stretch the fibers without fracture. We found that the nonwoven membrane had stress at break of 6.59 ± 1.67 MPa with a strain up to 130 ± 21% (Figure 2). The membranes did not deform

Figure 2. Stress−strain curve of the pristine PCL membrane under tensile force.

during gravity-driven filtration. Therefore, we attempted to stretch the nonwoven PCL membranes at different strain levels as oil/water emulsion separators without any additional support layer. As seen from the SEM images (Figure 3), the fibers were aligned in the stretching direction, and the pore shape was

Figure 3. SEM images of the electrospun PCL membranes stretched at different strain levels: (a) 0% strain, (b) 30% strain, (c) 50% strain, and (d) 80% strain. C

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Langmuir changed from polygon to petal-like shape due to the asymmetric elongation. The shrinkage in the perpendicular direction was obvious (Figure S2) with a closer packing of the fibers and a smaller pore gaps, although the fiber diameter did not change much, from 258 ± 50 nm (pristine) to 232 ± 48 nm (at 80% strain). It suggests that the applied stress changed the pore shape rather than the pore size. It is noted that the stretching was performed at room temperature to maintain the mechanical integrity of the membrane since PCL is a thermoplastic material that will flow above 60 °C. Because the random nature of the pore size and shape in the pristine membrane and the membrane was multilayered, we only compared the pore size before and after stretching in the top few layers using ImageJ to process the SEM images. Taking the maximum and minimum values of the brightness and contrast, we set the dark gray color as black for fibers and light gray color as white for voids. The details and adjusted SEM images are shown in Figure S2. We defined the pore size as the diameter of a circle that can fit inside a pore. Accordingly, the pore size of 0% strain membrane and 80% strain membrane was 359 ± 185 and 272 ± 153 nm, respectively. Likewise, we used ImageJ to measure the length and width of pores from the same SEM images to calculate the pore aspect ratio, which changed from 2.5 ± 1.9 (0% strain) to 5.3 ± 3.0 (80% strain), respectively. The anisotropy of the pores in the pristine membrane is possibly due to the shear stress experienced in electrospinning. We note that these values are not the real pore size and pore shape. They provide qualitative information on how the pore size and shape change as the strain increased. We then investigated the performance of PCL membranes for separation of the oil-in-water emulsions. 1% v/v hexadecane in water was used and the emulsifier was chosen based on its hydrophilic−lipophilic balance (HLB) value to ensure that this emulsion was stable throughout the filtration experiments. HLB is an expression of the relative attraction of an emulsifier for water and oil, determined largely by the chemical composition and ionization characteristics of a given emulsifier. HLB is scaled from 0 to 20. An HLB value of 0 corresponds to a molecule that is completely lipohilic, and a value of 20 corresponds to a molecule that is completely hydrophilic.28,29 We followed the HLB system developed by ICI Americas Inc., which guides the selection of surfactants to prepare stable emulsions. For a surfactant mixture A/B, the HLB value can be estimated as30

Figure 4. Filtration of the O/W (1% v/v hexadecane) emulsion: (a) filtration setup; (b−d) optical images of (b) the feed emulsion, (c) permeate from 0% strain PCL membrane, and (d) permeate from 80% strain PCL membrane.

membrane enhancing wetting selectivity between oil and water.13 When the emulsion was poured onto the membrane, the oil droplets were de-emulsified. Water phase selectively wet and spread on it and drained immediately while the oil phase was propelled from the membrane surface. For better visualization, we dyed the oil phase with Sudan red. The feed emulsions appeared pink and turbid due to the Tyndall effect. The micrometer-sized droplets in the feed emulsion could be seen under the optical microscope as shown in Figure 4b. After filtration, the permeates obtained from both the pristine and the stretched membranes appeared clear under the optical microscope (Figure 4c,d), suggesting an absence of particles larger than 40 nm in the permeates. We further characterized the emulsion droplet size in the permeate before and after filtration by DLS (Figure 5). As seen in Figure 5a, the feed emulsion had a trimodal size distribution peaked at 91 nm, 712 nm and 5.56 μm. After filtration, however, droplets with a diameter greater than 90 nm disappeared. By counting, average sizes of the droplets remained in the permeate were 28 and 18 nm from PCL membranes with 0% and 80% strain, respectively. It is clear that membranes with smaller pores and 80% strain could exclude ever smaller emulsion droplets, and it was achieved simply by stretching the membrane to improve the selectivity. It is noted that this method was successful because of the excellence mechanical properties of the electrospun PCL membrane. As aforementioned, volumetric flux often suffers when increasing pore selectivity by reducing pore size. The maximum volumetric flux Qmax is described as9

HLBmixture = (fraction A × HLBA ) + (fraction B × HLBB) (1)

To prepare stable emulsions from hexadecane in water, the HLB is 12. Therefore, we mixed 30% v/v Span 80 (HLB = 4.3) and 70% v/v Tween 80 (HLB = 15),31 resulting in HLB value of 11.8. We note that it is important to prepare stable emulsions for the separation studies, which has often been ignored in the literature. In filtration, higher flux can be achieved under high pressure, which could also deform the shape of the emulsion droplets such that the droplets squeeze through the pores smaller than the droplet size.32 Under continuous high pressure, the membrane could also disintegrate over time, increasing overall energy cost. Therefore, we performed gravity-driven filtration only in our experiments. The PCL membrane was fixed between two glass cylinders as shown in Figure 4a. The membrane was prewetted by water to provide continuous water channels that facilitated water transport throughout the

Q max =

ρP AγOWπ cos θadv,W(O) r 3 4η

t

(2)

where ρP is the number pore density of the membrane, A is the membrane area, γOW is the interfacial tension between oil and water, θadv,W(O) is the advancing oil contact angle on the membrane surface under water, η is the viscosity, r is the pore radius, and t is the thickness of the membrane. In the case of circular pores, volumetric flux could drop dramatically when the pore size is in nanoscale. As seen in Figure 6, for both 0% strain and 80% strain membranes, flux dropped exponentially at the beginning of the filtration but leveled at ∼60 L m−2 h−1 bar−1 for 0% strain membrane and ∼70 L m−2 h−1 bar−1 for 80% strain membrane. D

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Figure 5. Size distribution of O/W (1% v/v hexadecane) emulsion droplets before (the feed) and after (the permeate) filtration by volume (a) and by number count (b) using 0% strain and 80% strain membranes.

membrane was stretched to 80% strain. Therefore, we believe that reduced membrane thickness alone may not be sufficient to lead to the observed increase of flux. It is known that porosity also plays an important role in oil/water separation.33 ⎛ ρ ⎞ porosity = ⎜⎜1 − m ⎟⎟ × 100 ρb ⎠ ⎝

(3)

where ρm is the membrane density and ρb is the density of the bulk polymer. Accordingly, the porosity of 80% strain membrane was 88%, which is slightly higher than that of the pristine membrane (84% porosity). In addition, the change of pore shape might affect the water pathway. Through the nonwoven membrane, water would have random flowing directions whereas the flow of water though the stretched membrane is less tortuous. We believe that a coupled effect between decreased membrane thickness, increase of porosity, and alignment of the pores in the direction of water flow have contributed to the overall observed increase of volumetric flux. Lastly, we performed the COD tests, which are commonly used to measure the amount of organic compounds in water, to quantify the oil rejection efficiency. Both pristine and 80% strain membranes show the same high oil rejection efficiency, 99.0%.

Figure 6. Flux from PCL membranes stretched with 0% strain (blue) and 80% strain (red).

These values are relatively high when they are compared to the literature values. On a three-tier composite consisting of a nonporous poly(vinyl alcohol) (PVA) hydrogel/multiwall carbon nanotube (MWNT) as the top skin layer, an electrospun PVA as the midlayer (fibers with an average diameter of 130 nm and 82% porosity) and a conventional nonwoven poly(ethylene terephathalate) (PET) microfibrous membrane as the bottom support layer, a flux of ∼48 L m−2 h−1 bar−1 was reported for separating soybean oil (1350 ppm) from O/W emulsions.33 A flux of ∼70 L m−2 h−1 bar−1 was obtained from electrospun polyacrylonitrile (PAN) membrane (fibers with an average diameter of 153 nm and 85% porosity) coupled with a thin barrier layer of cross-linked PVA.34 Interestingly, the volumetric flux of 80% strain membrane was a little higher than that of the pristine one. The underwater−oil contact angle could affect the flux. If oil wets on the membrane surface, membrane pores will be blocked, resulting in a decrease of the flux.32 The underwater−oil (hexadecane) contact angles on the pristine PCL membrane and the 80% strain membrane were 149° and 138°, respectively. Since reduction of the oil contact angle on the membrane surface will increase the interactions between oil phase and water phase, reduction of flux is expected. However, it was not observed in our stretched PCL membranes. It is known that decreasing thickness and varying the pore shape could affect the flux.32 Wang et al.33 showed that while the thickness of the electrospun PAN membrane was increased from 16 to 105 μm (6.6-fold), the water flux was decreased only by 1.7-fold. It is very different from the conventional membranes which often have dead-end pores. In contrast, electrospun membranes have interconnected pores, which is important to maintain water pathways. In our system, the membrane thickness decreased from 32 to 29 μm after the

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CONCLUSION We prepared hydrophilic nonwoven membranes by mixing Tween 80 and PCL in the mixed solvent, acetic acid and formic acid (3:1 v/v), followed by electrospinning. The obtained PCL membranes had high porosity (84%) and excellent mechanical strength as free-standing separators. The membranes could filtrate the emulsion droplets with size greater than 1 μm completely, leading to clear permeates. In order to separate submicrometer sized emulsions, we stretched the PCL membrane uniaxially to different strain levels to narrow the pore gap and increase the aspect ratio of the pores. At 80% strain, the aspect ratio of the pores was increased to 5.3 ± 3.0 while the porosity remained nearly unchanged. The 80% strain membrane could exclude O/W emulsion droplets with size as small as 18 nm. The flux from 80% strain membrane was maintained at 70 L m−2 h−1 bar−1 vs 60 L m−2 h−1 bar−1 from the pristine membrane. Both membranes show oil rejection efficiency as high as 99.0%. The study presented here via stretching of nonwoven fibers is simple and effective. It offers new insights of how pore shape could influence the oil/water emulsion separation efficiency and flux. We believe that both the pore geometry/size and the 3D network nature of the E

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(10) Holdich, R.; Kosvintsev, S.; Cumming, T.; Zhdanov, S. Pore design and engineering for filters and membranes. Philos. Trans. R. Soc., A 2006, 364 (1838), 161−174. (11) Lalia, B. S.; Kochkodan, V.; Hashaikeh, R.; Hilal, N. A review on membrane fabrication: Structure, properties and performance relationship. Desalination 2013, 326, 77−95. (12) Kaur, S.; Sundarrajan, S.; Rana, D.; Sridhar, R.; Gopal, R.; Matsuura, T.; Ramakrishna, S. Review: the characterization of electrospun nanofibrous liquid filtration membranes. J. Mater. Sci. 2014, 49 (18), 6143−6159. (13) Ge, D. T.; Yang, L. L.; Wang, C. B.; Lee, E.; Zhang, Y. Q.; Yang, S. A multi-functional oil-water separator from a selectively pre-wetted superamphiphobic paper. Chem. Commun. 2015, 51 (28), 6149−6152. (14) Xue, Z.; Wang, S.; Lin, L.; Chen, L.; Liu, M.; Feng, L.; Jiang, L. A Novel Superhydrophilic and Underwater Superoleophobic Hydrogel-Coated Mesh for Oil/Water Separation. Adv. Mater. 2011, 23 (37), 4270−4273. (15) Zhang, W.; Zhu, Y.; Liu, X.; Wang, D.; Li, J.; Jiang, L.; Jin, J. Salt-Induced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA-g-PVDF Membranes for Effective Separation of Oilin-Water Emulsions. Angew. Chem., Int. Ed. 2014, 53 (3), 856−860. (16) Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A. Hygro-responsive membranes for effective oil−water separation. Nat. Commun. 2012, 3, 1025. (17) Wang, X. F.; Yu, J. Y.; Sun, G.; Ding, B. Electrospun nanofibrous materials: a versatile medium for effective oil/water separation. Mater. Today 2016, 19 (7), 403−414. (18) Wang, X.; Zhang, K.; Yang, Y.; Wang, L.; Zhou, Z.; Zhu, M.; Hsiao, B. S.; Chu, B. Development of hydrophilic barrier layer on nanofibrous substrate as composite membrane via a facile route. J. Membr. Sci. 2010, 356 (1−2), 110−116. (19) Yoon, Y.; Hsiao, B. S.; Chu, B. High flux ultrafiltration nanofibrous membranes based on polyacrylonitrile electrospun scaffolds and crosslinked polyvinyl alcohol coating. J. Membr. Sci. 2009, 338 (1−2), 145−152. (20) Zhang, P.; Tian, R. P.; Lv, R. H.; Na, B.; Liu, Q. X. Waterpermeable polylactide blend membranes for hydrophilicity-based separation. Chem. Eng. J. 2015, 269, 180−185. (21) Cho, S. J.; Jung, S. M.; Kang, M.; Shin, H. S.; Youk, J. H. Preparation of hydrophilic PCL nanofiber scaffolds via electrospinning of PCL/PVP-b-PCL block copolymers for enhanced cell biocompatibility. Polymer 2015, 69, 95−102. (22) Srisook, T.; Vongsetskul, T.; Sucharitakul, J.; Chaiyen, P.; Tangboriboonrat, P. Immobilization of 3-hydroxybenzoate 6-hydroxylase onto functionalized electrospun polycaprolactone ultrafine fibers: A novel heterogeneous catalyst. React. Funct. Polym. 2014, 82, 41−46. (23) Shahin, M.; Hady, S. A.; Hammad, M.; Mortada, N. Novel Jojoba Oil-Based Emulsion Gel Formulations for Clotrimazole Delivery. AAPS PharmSciTech 2011, 12 (1), 239−247. (24) Woodard, L. F.; Jasman, R. L. Stable Oil-in-Water Emulsions Preparation and Use as Vaccine Vehicles for Lipophilic Adjuvants. Vaccine 1985, 3 (2), 137−144. (25) Moon, J. H.; Kim, A. J.; Crocker, J. C.; Yang, S. HighThroughput Synthesis of Anisotropic Colloids via Holographic Lithography. Adv. Mater. 2007, 19 (18), 2508−2512. (26) Lowery, J. L.; Datta, N.; Rutledge, G. C. Effect of fiber diameter, pore size and seeding method on growth of human dermal fibroblasts in electrospun poly(ε-caprolactone) fibrous mats. Biomaterials 2010, 31 (3), 491−504. (27) Kalayci, V. E.; Patra, P. K.; Buer, A.; Ugbolue, S. C.; Kim, Y. K.; Warner, S. B. Fundamental investigations on electrospun fibers. J. Adv. Mater.-Covina 2004, 36 (4), 43−47. (28) Griffin, W. C. Classification of surface-active agents by HLB. J. Soc. Cosmet. Chem. 1949, 1 (5), 311−326. (29) Griffin, W. C. Calculation of HLB values of non-ionic surfactants. J. Soc. Cosmet. Chem. 1954, 5, 249−256. (30) ICI Americas, I. The HLB System: A Time-saving Guide to Emulsifier Selection, 1984.

electrospun membranes we fabricate play important roles to allow separation of submicrometer emulsions without sacrificing the separation flux.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01138. Optical images of a water droplet spreading over time on the PCL membrane treated with Tween 80, apparatus to stretch the PCL membrane, SEM images of 0% strain and 80% strain PCL membranes for pore size and shape evaluation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +1-215-573-2128; Tel +1-215-898-9645 (S.Y.). ORCID

Shu Yang: 0000-0001-8834-3320 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported in part by National Academy of Keck Future Initiative (NAKFI) grant and Xerox Foundation grant. W.P. acknowledges the support by the Development and Promotion of Science and Technology Talents Project (Royal Government of Thailand scholarship). The Singh Center for Nanotechnology is acknowledged for use of SEM.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b01138 Langmuir XXXX, XXX, XXX−XXX